1. Field of the Invention
The present invention is directed to methods of preparation that effectively disperse carbon nanotubes into polymer matrices, and the novel nanocomposites that result therefrom.
2. Description of the Related Art
Since carbon nanotubes (CNTs) were discovered in 1991 (S. Iijima, Nature 354 56, 1991), significant interest has been generated due to their intrinsic mechanical, electrical, and thermal properties (P. M. Ajayan, Chem. Rev. 99 1787, 1999). Early studies focused on CNT synthesis and theoretical prediction of physical properties. Due to the recent development of efficient CNT synthesis (A. Thess et al., Science 273 483, 1996) and purification procedures (A. G. Rinzler et al., Appl. Phys. A 67 29, 1998), some applications have been realized. However, these applications have relied on the use of pure CNTs, not nanocomposites. Examples include a carbon nanoprobe in scanning probe microscopy (S. S. Wong et al., J. Am. Chem. Soc. 120 603, 1998), single wall carbon nanotube (SWNT) transistor (S. J. Tans et al., Nature 393 49, 1998), and field emission display (Q. H. Wang et al., Appl. Phys. Lett. 70 3308, 1997). There have been very few reports on the development of nanocomposites using CNTs as reinforcing inclusions in a polymer matrix primarily because of the difficulty in dispersing the nanotubes. This difficulty is partially due to the non-reactive surface of the CNT. A number of studies have concentrated on the dispersion of CNTs, but complete dispersion of the CNTs in a polymer matrix has been elusive due to the intrinsicly strong van der Waals attraction between adjacent tubes. In practice, attempts to disperse CNTs into a polymer matrix leads to incorporation of agglomerates and/or bundles of nanotubes that are micron sized in thickness and, consequently, they do not provide the desired and/or predicted property improvements. Most of the dispersion related studies have focused on modifying the CNT surface chemistry. Many researchers have studied the functionalization of CNT walls and ends. One example is fluorination of CNT surfaces (E. T. Mickelson et al., J. Phys. Chem. 103 4318, 1999), which can subsequently be replaced by an alkyl group to improve the solubility in an organic solvent. Although many researchers have tried to functionalize CNT ends and exterior walls (as a means to increase solubility) by various approaches such as electrochemistry and wrapping with a functionalized polymer, the solubility of these modified tubes was very limited. Other methods of CNT modification include acid treatment (i.e. oxidation) and use of surfactants as a means of improving solubility and compatibility with organic polymers. It has been noted that modifications of the nanotube chemical structure may lead to changes in intrinsic properties such as electrical conductivity (X. Gong et al., Chem. Mater., 12 1049, 2000). Ultrasonic treatment has also been used as a means to disperse CNTs in a solvent. Upon removal of the sonic force, the tubes agglomerate and settle to the bottom of the liquid.
Individual SWNTs can exhibit electrical conductivity ranging from semi-conductor to metallic depending on their chirality, while the density is in the same range of most organic polymers (1.33-1.40 g/cm3). In the bulk, they form a pseudo-metal with a conductivity of approximately 105 S/m (Kaiser et al., Physics Reviews B, 57, 1418 1998). The conductive CNTs have been used as conductive fillers in a polymer matrix to enhance conductivity, however the resulting nanocomposites exhibited little or no transparency in the visible range (400-800 nm). Coleman et al., (Physical Review B, 58, R7492, 1998) and Curran et al., (Advanced Materials, 10, 1091, 1998) reported conjugated polymer-CNT composites using multi-wall CNTs, which showed that the percolation concentration of the CNTs exceeded 5 wt %. The resulting nanocomposites were black with no transparency in the visible region. Shaffer and Windle (Advanced Materials, 11, 937, 1999) reported conductivity of a multi-wall CNT/poly(vinyl alcohol) composite, which also showed percolation above 5 wt % nanotube loading and produced a black nanocomposite. The same group (J. Sandler, M. S. P. Shaffer, T. Prasse, W. Bauhofer, K. Schulte, and A. H. Windle, Polymer 40, 5967, 1999) reported another multi-wall CNT composite with an epoxy, which achieved percolation below 0.04 wt %. An optical micrograph of the CNT/epoxy composite was reported, which revealed that the CNT phase was separated from the epoxy resin, showing several millimeters of resin-rich domains. The dispersion of CNTs in this material was very poor. This agglomeration of CNTs in selected areas in the composite could explain the high conductivity observed since it provides the “shortest path” for the current to travel. Preliminary measurements of the conductivity of a CNT/poly(methyl methacrylate) (PMMA) composite were measured on a fiber (R. Haggenmueller, H. H. Gommans, A. G. Rinzler, J. E. Fischer, and K. I. Winey, Chemical Physics Letters, 330, 219, 2000). The level of conductivity was relatively high (1.18×10−3 S./cm) at 1.3 wt % SWNT loading. However, the optical transparency in the visible range was not determined for the fiber sample. The mechanical properties of these fibers were much less than the predicted value, which implies that the CNTs were not fully dispersed.
The present invention is directed to methods of preparation that overcome the shortcomings previously experienced with the dispersion of CNTs in polymer matrices and the novel compositions of matter produced therefrom. The resulting nanocomposites exhibit electrical conductivity, improved mechanical properties, and thermal stability with high retention of optical transparency in the visible range.